Research ArticleThe QoS Indicators Analysis of IntegratedEUHT Wireless Communication System Based on UrbanRail Transit in High-Speed Scenario

Xiaoxuan Wang ,1 Hailin Jiang,2 Tao Tang,1 and Hongli Zhao1

1State Key Laboratory of Rail Traffic Control and Safety, Beijing Jiaotong University, Beijing 100044, China2National Engineering Research Center of Rail Transportation Operation and Control System, Beijing Jiaotong University,Beijing 100044, China

Correspondence should be addressed to Xiaoxuan Wang; wangxiaoxuan@bjtu.edu.cn

Received 2 November 2017; Accepted 15 February 2018; Published 3 April 2018

Academic Editor: Yan Zhang

Copyright © 2018 XiaoxuanWang et al.This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Nowadays, in urban rail transit systems, train wayside communication system uses Wireless Local Area Network (WLAN) aswireless technologies to achieve safety-related information exchange between trains and wayside equipment. However, accordingto the high speed mobility of trains and the limitations of frequency band,WLAN is unable to meet the demands of future intracityand intercity rail transit. And although the Time Division-Long Term Evolution (TD-LTE) technology has high performancecompared with WLAN, only 20MHz bandwidth can be used at most. Moreover, in high-speed scenario over 300 km/h, TD-LTEcan hardly meet the future requirement as well. The equipment based on Enhanced Ultra High Throughput (EUHT) technologycan achieve a better performance in high-speed scenario compared with WLAN and TD-LTE. Furthermore, it allows using thefrequency resource flexibly based on 5.8GHz, such as 20MHz, 40MHz, and 80MHz. In this paper, we set up an EUHT wirelesscommunication system for urban rail transit in high-speed scenario integrated all the traffics of it. An outdoor testing environmentin Beijing-Tianjin High-speed Railway is set up to measure the performance of integrated EUHT wireless communication systembased on urban rail transit. The communication delay, handoff latency, and throughput of this system are analyzed. Extensivetesting results show that the Quality of Service (QoS) of the designed integrated EUHT wireless communication system satisfiesthe requirements of urban rail transit system in high-speed scenario. Moreover, compared with testing results of TD-LTE whichwe got before, the maximum handoff latency of safety-critical traffics can be decreased from 225ms to 150ms. The performanceof throughput-critical traffics can achieve 2-way 2Mbps CCTV and 1-way 8Mbps PIS which are much better than 2-way 1MbpsCCTV and 1-way 2Mbps PIS in TD-LTE.

1. Introduction

With the rapid development of the city size, the urban railtransit becomes the main trip mode of human beings whichcould provide safe, punctual passenger services within urbanareas and intercity. In order to release the traffic pressurein urban area, the transit speed and capacity need to beimproved along with more and more urban population. So,an automated train control system called communication-based train control (CBTC), which uses high capacity andbidirectional train-wayside communication, becomes oneof the key subsystems in urban rail transit systems toguarantee the safe operation of rail vehicles [1]. According

to the open standards and available commercial-off-the-shelf equipment [2], the WLANs are often used as thetrain-wayside communication in urban rail transit, such asSingapore North-East Line from Alstom [3] and BeijingMetro Line 10 form Siemens. However, with the develop-ment of urban rail transit, such as high-speed operationand integrated services including CBTC information, trainstate monitor information, passenger information systems(PIS) video, closed-circuit television (CCTV) video, andemergency text, WLAN cannot meet the developed safety-critical and throughput-critical requirements of urban railtransit and QoS of wireless communication system due toincreasing operation density.

HindawiWireless Communications and Mobile ComputingVolume 2018, Article ID 2359810, 9 pageshttps://doi.org/10.1155/2018/2359810

2 Wireless Communications and Mobile Computing

Although TD-LTE obtains some improvement in com-munication performance comparingwithWLAN, the perfor-mance of TD-LTE is degraded greatly in high-speed scenarioaround 300 km/h, such as reduced throughput and largecommunication delay [4]. Therefore, the existing technologycan hardly guarantee the strict QoS requirements of urbanrail transit systems in high-speed scenario over 300 km/h.

Hence, a new technology which can solve the problemsin high-speed scenario needs to be sought. Fortunately, theseproblems can be addressed by a new technology namedEUHT. EUHT is a novel wireless communication technologybased on 802.11ac which can achieve the requirements of5G [5]. The highest throughput of this system can reach3.48Gbps which is much larger than the TD-LTE. Theanalysis of throughput is described in Section 3. And thecommunication delay can be lower than 10ms normally.Furthermore, other advantages of EUHT are set as follows:

(i) Frame length of EUHT can be configured accordingto different scenarios from 0.5ms to 10ms and EUHTdesigned configurable pilot for effective channel esti-mation for high speed scenario.

(ii) EUHT physical frame is the first to use the self-contained structure, that is, uplink and downlinkresource allocation. downlink data transmission andthe uplink acknowledgment transmission are com-pleted in the same frame, which is considered to beone of the key technologies of 5G by Qualcomm.Theself-contained frame structure can greatly reduce thesystem latency.

(iii) EUHT has designed a special sequence at the headerof frame to enable the receiver to achieve a moreefficient and reliable synchronization process. Thereceiver can perform frame detection, automaticgain control (AGC), and other operations basedon the EUHT header without any other auxiliarysynchronization sources (such as base station, GPS).Through the optimization design of the synchroniza-tion sequence, the receiver can achieve a reliableestimate of the large frequency deviation at lowcost, thus greatly reducing the requirement of crystalperformance to +/−20 ppm.

Due to these advantages, EUHT has a better performancethan existing WLAN and TD-LTE technology in high-speedscenario up to 500 km/h.

At present, there are some researches about the com-munication system based on 802.111ac. Authors proposedelayed dynamic bandwidth channel access scheme withvirtual primary channel reservation in [6]. This schememitigates the performance bottleneck problem as well as ascalability problem. In [7], an antenna allocation scheme fora full-duplex communication is proposed in IEEE 802.11acWLAN. A set of results from a performance evaluationstudy of wireless communication based on the IEEE 802.11acstandard are proposed to study the performances of IEEE802.11ac communication system in [8]. These related worksmake some contributions on using the 802.11ac technologyin public wireless communication networks, but they do not

consider the integrated urban rail transit traffics in high-speed scenario.

Recently, substantial works have been done on thewireless communication issues in the railway environment.Authors of [9] use the cross layer Reliable Mobility PatternAware (RMPA) handover strategy to improve handoverperformance for broadbandwireless communication in high-speed railway. Not only do He et al. in [10] test the broadbanddelay cluster performance at 2.4GHz in subway tunnel, butalso the analysis of the results are given. In [11], a fieldtest has been done in Madrid subway to build the radiochannel in tunnels at 2.4GHz. The authors in [12] giveus a cross-layer admission control scheme for high-speedrailway communication system. Zhu et al. design a cross-layerhandoffmethod for CBTCwireless communication system in[13].

Although these aforementioned works consider theCBTC performance under different railway wireless commu-nication environment, few of them study the integrated urbanrail transit traffics in the same environment. Moreover, theworking frequency of related works is all based on 2.4GHz orlower. There are few studies that consider the high workingfrequency communication system in high-speed scenario.In this paper, an integrated EUHT wireless communicationsystem based on urban rail transit in high-speed scenario isdesigned, which works in the 5.8GHz and includes all thetraffics in CBTC systems.

In this paper, we firstly study the integrated EUHTwireless communication system based on urban rail transitin high-speed scenario, which works in the 5.8GHz. Then,the theoretical calculation of communication delay withouthandoff, handoff latency, and throughput in EUHT wire-less communication system based on urban rail transit aregiven. In order to get the real integrated EUHT wirelesscommunication system performance, we set up an outdoortest scenario in Beijing-Tianjin High-Speed Railway. The testresults show that the QoS performance of integrated EUHTwireless communication system based on urban rail transitnot only satisfies the requirements of safety-critical trafficsand throughput-critical traffics in high-speed scenario well,but also has greatly performance improvement comparingwith TD-LTE system.

The remainder of this paper is organized as follows.Section 2 describes the integrated EUHT wireless commu-nication system based on urban rail transit. In Section 3,the theoretical calculation of communication delay withouthandoff, handoff latency, and throughput are given. The testscenario in Beijing-TianjinHigh-SpeedRailway is introducedin Section 4. The the requirements of all the traffics in CBTCsystems and test results are presented in Section 5. Finally, theconclusion is given.

2. The Integrated EUHT WirelessCommunication System Based onUrban Rail Transit

As we can see in Figure 1, the integrated EUHTwireless com-munication system based on urban rail transit includes the

Wireless Communications and Mobile Computing 3

EDU EDU EDU

EBU EBUEBUEBU

EAT

Controlcenter

equipment

Trainstation

equipment

Waysideequipment

Onboardequipment

EAT EAT EAT

ESU Onboard networkEAUESUEAU

Backbone network

ECC EDCPIS serverCCTV serverRadio dispatch

ServerTrain state

monitor centerCBTC control

center

ATS ATP CI

Onboard equipment

OnboardCBTC

equipment

Radiodispatch

CameraCamera

LCDcontroller PC

Switch

Figure 1: System architecture of EUHT wireless communication system based on urban rail transit.

control center subsystem, train station subsystem, tracksidesubsystem, and on-board subsystem.

The main task of control center subsystem is to useplenty of equipment such as EUHT Control Center (ECC),EUHT Data Center (EDC), CBTC control center, train statemonitor center, CCTV server, radio dispatch server, andPIS server to control and manage the integrated CBTCsystem. Moreover, the backbone network is used to pro-vide the wired communication link between control cen-ter subsystem and train station subsystem. As there aretrains operating in the management area of any trainstation, the train station subsystem will charge the com-munication with all trains. The track-side subsystem con-sists of the EUHT Base-Station Unit (EBU) and EUHTAntenna (EAT). The EBU is the base station of the EUHTsystem and connected to EAT using optical fiber. TheEAT exchanges the information with on-board subsystemthrough air interface. The on-board subsystem can bedivided into EUHT Access Unit (EAU), EUHT ServiceUnit (ESU), PIS screens, CCTV cameras, onboard CBTCequipment, train radio dispatch equipment, and train statesensors. All these types of equipment are connected byswitches.

Moreover, in order to verify whether the integratedEUHT wireless communication system can satisfy the QoSrequirements of different traffics in urban rail transit, thetheoretical analysis is shown in Section 3 and the details abouttest scenario and procedure are presented in Section 4.

EAU Frameadjustment EBU AGW

1ＧＭ

1ＧＭ

1ＧＭ

1ＧＭ

1ＧＭ

1ＧＭ 0.5 ＧＭ

0.5 ＧＭ

0.5 ＧＭ

Tf-＞Ｆ ms

Ta-＞Ｆ ms

Tf-ＯＦ ms

Ta-ＯＦ ms

Tf-＞Ｆ ms

Figure 2: Transmission delay without handoff.

3. The Theoretical Analysis of IntegratedEUHT Wireless Communication System

In this section, the theoretical analysis of communicationdelay, handoff latency, and throughput are analyzed.

3.1. Communication Delay without Handoff. The communi-cation delay without handoff is shown in Figure 2, where𝑇𝑎-dl and𝑇𝑎-ul are the downlink and uplink averageAutomatic

4 Wireless Communications and Mobile Computing

EAU EBU-S EBU-D

(d) HO-REQ

(f) HO-CMD(e) Handoff decision

Handoffexecution

Send PN code

Resource allocation CCH

RA-REQ

RA-RSP

(g) EAU disconnect with EBU-S

(a) HM-REQ

(b) HM-RSP

(c) HM-REPHandoff

measurement

Handoffpreparation

Handoffcompletion

(h) EAU connect with EBU-D

SBC-REQ

SBC-RSP

ACK

RCC

Figure 3: The handoff procedure of EUHT system.

Repeat Request-Round Trip Time (ARQ-RTT), respectively,and Tf-dl and Tf-ul are the average downlink and uplink frameadjustment time, respectively. With the analysis, the EUHTcommunication delay Commdelay is shown as follows:

Commdelay = 1 + 𝑇𝑓 + 1 + 𝑝 ∗ 𝑇𝑎, (1)

where𝑇𝑎 and𝑇𝑓 are the average ARQ-RTT and frame adjust-ment time.

For our test, the theoretical calculation result with typicalparameters of communication delay without handoff andretransmission is 4–8ms. However, there is no ideal wirelessenvironment of the high speed scenario around 300 km/h.Therefore, the real communication delay is greater than thisresult.

3.2. Handoff Latency. In order to make the best of thefrequency resources and eliminate the intercell interference,the InterfrequencyHandoff (IFHO) is selected by EUHT.Thetheoretical analysis of it is set below.

As is shown in Figure 3, the handoff procedure includeshandoff measurement, handoff preparation, handoff execu-tion, and handoff completion.The handoffmeasurement and

preparation phases are performed without EBU-Destination(EBU-D), so that themessages in these two phases are directlyexchanged among the EAU and the EBU-Service (EBU-S).At the beginning, when the Radio Signal Strength Indicator(RSSI) of EBU-S has been lower than the measurementthreshold for a certain period set before, the EAU will sendthe Handoff Measurement Request (HM-REQ) to EBU-Sto apply to starting the handoff measurement. Then, whenthe EAU receives the Handoff Measurement Response (HM-RSP), it will measure the RSSI of ESU-S and EBU-D andreport the information to EBU-S using a Handoff Measure-ment Report (HM-REP). When RSSI of the EBU-D has beenstronger than that of the EBU-S for a certain period, theEAU will decide to execute handoff and initiate the handoffpreparation phase by sending a Handoff Request (HO-REQ)to the EBU-S. AHO-REQ includes EAU context information,QoS parameters, and the information of EBU-D candidates.EBU-S selects an EBU-D from the candidates and replies tothe EAU with a Handoff Command (HO-CMD). Therefore,the latency for the handoff preparation phase is representedas follows:

𝑇HO-Prep = 2 ∗ 𝑇EAU-EBU-S + 𝑇con1 + 𝑇dec1, (2)

Wireless Communications and Mobile Computing 5

S-P

DL-TCHCCH DL-TCH UL-TCH

L-PSICH

DL-SCH

UL-SCH

DGI

UGI

UL-SRCHUL-RACH

Time

Downlink period Uplink period

Figure 4: The frame structure of EUHT system.

where 𝑇EAU-EBU-S indicates the transmission delay betweenthe EAU and EBU-S, 𝑇con1 is the latency of the measure-ment frequency conversion of EAU, and 𝑇dec1 representsthe processing delay of the EBU-S, where 𝑇dec1 includes theprocessing of EBU-D selection and handoff decision.

The handoff execution phase is most critical on thehandoff performance due to the handoff interruption/dis-connection that occurs. The handoff execution phase istriggered by the EBU-S sending a HO-CMD to the EAU.Upon reception of the HO-CMD, the EAU disconnects fromthe EBU-S and changes the working frequency to be thesame as EBU-D. Then, EAU sends Pseudo-Noise (PN) codeto the assigned EBU-D through the RandomAccess Channel(RACH) and attempts to initiate the Random Access (RA)process. Next, As the EBU-D receives the PN code, it willallocate the Control Channel (CCH) resources for EAU andEAU will send RA-REQ message through the CCH. Afterreceiving RA-REQ message and RAU MAC, the EBU-D willreply the RA-RSP to EAU. Then, in order to inform EAUcapability parameters to EBU-D, EAU sends the Station BasicCapability (SBC-REQ) to EBU-D. As EBU-D receives therequest, it will compare the capability parameters of EAU andEBU-D and reply the SBC-RSP which includes the capabilityparameters supported by EBU-D, the allocated EAU ID, andscheduling information to EAU. The latency for the handoffexecution phase 𝑇HO-Exe is represented as

𝑇HO-Exe = 6 ∗ 𝑇EAU-EBU-D + 𝑇con2 + 𝑇dec2, (3)

where 𝑇EAU-EBU-S indicates the transmission delay betweenthe EAU and EBU-D, 𝑇con1 is the latency of the measure-ment frequency conversion of EAU, and 𝑇dec1 representsthe processing delay of the EBU-S, where 𝑇dec1 includes theprocessing of EBU-D selection and handoff decision.

During the handoff completion phase, when the EAUcompletes the RA process, the EAU informs the EBU-Dby sending a RA Connection Complete (RCC) and theEBU-D replies to the EAU with an ACK. Finally, theEAU connects with EBU-D and transmits traffics throughtheUplink/Downlink-TransmissionChannel (UL/DL-TCH).The handoff completion latency is expressed as

𝑇HO-Comp = 2 ∗ 𝑇EAU-EBU-D + 𝑇ACK, (4)

where 𝑇ACK is the processing delay of the EBU-D, whichincludes the processing of RRC and sending ACK.

So, the total handoff latency is presented below:

𝑇HO = 2 ∗ 𝑇EAU-EBU-S + 8 ∗ 𝑇EAU-EBU-D + 𝑇con1 + 𝑇con2

+ 𝑇dec1 + 𝑇dec2 + 𝑇ACK.(5)

In our test, given the typical parameters, the handofflatency without failure is about 20ms, and with high speedscenario reaching up to 300 km/h and complex environmentin the field test, the real handoff latency is greater than this.

3.3. Throughput. In this subsection, the throughput withoverhead needs to be calculated at first. According to thecharacteristic of 802.11ac [14], the single cell throughput ofEUHT system mainly depends on system bandwidth, lengthof OFDM symbol, and Modulation and Coding Scheme(MCS). Thus, the peak data rate is

Throughput =𝑁DBPS𝐿, (6)

where𝑁DBPS is the number of data bits per symbol and𝐿 is thelength of OFDM symbol, which is 14.4 𝜇s. Moreover, 𝑁DBPSis represented as

𝑁DBPS = 𝑁BPSCS ∗ 𝑅 ∗ 𝑁SS ∗ 𝑁subcarrier, (7)

where 𝑁BPSCS indicates the modulation order, 𝑅 is the coderate, 𝑁SS represents the number of spatial streams, and𝑁subcarrier is the number of subcarrier.

However, as is shown in Figure 4, the transmission frameof EUHT system consists of Short-Preamble (S-P), Long-Preamble (L-P), System Information Channel (SICH), CCH,UL/DL-TCH, Uplink/Downlink-Search Channel (UL/DL-SCH), Uplink/Downlink-Guard Interval (U/D-GI), Uplink-Scheduling Request Channel (UL-SRCH), and Uplink-RACH (UL-RACH). Because UL/DL-TCH can only beused to transmit train-ground communication traffics inone transmission frame, the uplink and downlink peakthroughput need to be calculated with data bits of carryingwithout overhead which contains S/L-P overhead, SICHoverhead, CCH overhead, UL/DL-SCH overhead, U/D-GIoverhead, UL-SRCH overhead, UL-RACH overhead, andpilot frequency (PF) overhead, over a single wireless frametime.

In this paper, the length of one frame is 2ms, which totallyhave 139 OFDM symbols and the UL/DL configuration is1 : 1.2. Moreover, in our test, S/L-P, SICH, CCH, UL/DL-SCH,U/D-GI, UL-SRCH, UL-RACH, and PF occupy 2, 1, 4, 2, 8,

6 Wireless Communications and Mobile Computing

PC11

ECC

PC12

PC13

PC14

EDC

SWITCH

EDU

Machineroom

PC21 PC22 PC23 PC24

ESU

Divider

Onboard EAU

BeijingSouth

RailwayStation

WuqingRailwayStation

TianjinRailwayStation

Figure 5: Test scenario of Beijing-Tianjin High-Speed Railway.

1, 2, and 9 OFDM symbols in one frame, respectively. So, thetotal overhead is 29 OFDM symbols in one frame. The totaloverhead of EUHT system in one frame is represented as

OHtotal = OHS/L-P +OHSICH +OHCCH

+OHUL/DL-SCH +OHU/D-GI +OHUL-SRCH

+OHUL-RACH +OHPF.

(8)

According to the analysis in this subsection, the downlinkthroughput of EUHT which does not include the overheadcan be shown as

Throughpu𝑡DL = Throughput ∗ (1 −OHtotal) ∗1.2

2.2. (9)

The uplink throughput of EUHTwithout overhead can bewritten as

Throughpu𝑡UL = Throughput ∗ (1 −OHtotal) ∗1

2.2. (10)

For our field test, the bandwidth and maximum MCSare 80MHz and 21. So, the maximum uplink and downlinkthroughput are 179.06Mbps and 214.87Mbps, respectively.But with high speed scenario reaching up to 300 km/h andcomplex environment in the field test, the real throughput islower than this.

4. Test Scenario and Parameters

In this section, a field test scenario has been done in theBeijing-Tianjin High-Speed Railway to make the testing

results more accurate than that in the laboratory. The testscenario is made up of three parts, obviously, the high-speedrailway, machine room plant, and on-board space. As isshown in Figure 5, the Beijing-Tianjin High-Speed railwayis located between Beijing and Tianjin and has a length of115.2 km. It starts from Beijing South Railway Station andends at Tianjin Railway Station by way of Wuqing RailwayStation.Thehigh-speed railway ismade up of elevated railwayexcept the area around railway station. So, in most area of therailway, there is no tall building which can affect the wirelesstransmission.

In this field test, in order to guarantee the Quality ofCoverage (QoC), 109 EBUs are deployed along the railway intotal. The distance between every two EBUs is about 1 km.So, the average RSSI received by onboard EAU is above−90 dBm which can meet the demand of urban rail transitwireless communication system. Moreover, every adjacentEBU works at different frequency point due to IFHO. Thus,to make the different traffics meet the QoS requirementdescribed in Table 2, we allocate 160MHz for the wholesystem. Half of the EBUs run in 5560MHz–5640MHzfrequency band, and others run in 5725MHz–5805MHzfrequency band. These two frequency bands are all pos-sibly used for high-speed railway and urban rail transitsystem. Furthermore, because of less use of the frequencyresources in 5.6GHz–5.8GHz, there is little wireless inter-ference in this frequency band along the railway. So, theradio free wave is used in wireless communication sys-tem instead of leaky cable and leaky waveguide. What ismore, Table 1 shows the other configurations of this testscenario.

Wireless Communications and Mobile Computing 7

Table 1: The configurations in Beijing-Tianjin High-Speed Railway.

Parameters ValueEBU TX power 17 dBmTrack-side antenna gain 17 dBFrequency point 120 (5600MHz), 153 (5765MHz)Working bandwidth 80MHzHandoff model IFHO (interfrequency handoff)Wireless coverage Free waveMaximum train speed 300 km/hOnboard antenna height 3mTrack-side antenna height 5m

As we can see in Figure 5, in the machine room, ECCis used for controlling the EUHT network. All the data areanalyzed and processed by EDC. The EDU is connected tothe wayside equipment with optical fiber. Four PCs are theanalog servers for CBTC, train state monitor, PIS, CCTV,and emergency text, respectively. There are also EAU usedfor onboard network access and ESU used for analyzing andprocessing the data received by EAU, and four PCs connectedwith ESU are the analog on-board servers for CBTC, trainstate monitor, PIS, CCTV, and emergency text, respectively.Therefore, one-way CBTC traffic (200Kbps), two-way CCTVtraffic (2Mbps), one-way PIS traffic (8Mbps), one-way emer-gency text traffic (200Kbps), and one-way train statemonitortraffic (200Kbps) are set to be transmitted between the PCslocated at the machine room plant and on-board space.

The test tool used for test is Ixchariot software which cansimulate different urban rail transit traffics. There are alsodifferent IP and traffics set for different pair of PCs. PC11 andPC21 are for CBTC traffic. PC12 and PC22 are for train statemonitor traffic. PC13 and PC23 are for CCTV traffic. PC14and PC24 are for PIS traffic and emergency text traffic.

5. Test Results and Discussions

As is shown in our previous works, the requirements of allthe traffics in CBTC systems, such as communication delay,handoff latency, and throughput, are much stricter than thatin public wireless networks. Thus, the communication delayand handoff latency must be stipulated less than the train-wayside communication period which is 200ms in the tra-ditional CBTC systems; otherwise the trains may experiencesome serious accidents [15]. Furthermore, the throughput isrelated to the performance of the high throughput demandtraffics, such as PIS and CCTV.

Table 2 shows the different requirements for differenttraffics in integrated urban rail transit [16].Therefore, in orderto ensure the safe operation of the trains on the line, thetransmission delay of safety-critical traffics, such as CBTCinformation, train state monitor, and emergency text, shouldbe stipulated less than 150ms through wireless and wiredtransmission whether the handoff happens or not. And thelatency requirement of the other nonsafety critical traffics isless than 500ms. In the requirement of throughput, at least100 kbps should be allocated to CBTC traffic both in uplink

Elapsed time (minutes)0 5 10 15 20 25 30

Late

ncy

(s)

0

0.05

0.1

0.15

0.2

CBTCTrain state monitorEmergency text

Figure 6: Test result of communication delay and handoff latencybased on EUHT.

and in downlink due to the bidirectional transmission of it.And according to the unidirectional transmission of trainstate monitor and emergency text, there are total 200 kbpsallocated to these two traffics. Furthermore, the throughput-critical traffics, such as PIS and CCTV, are required 8Mbpsin downlink and 4Mbps in uplink at least, respectively.

Therefore, in this section, some testing results about com-munication delay, handoff latency, and throughput are shownto describe whether the integrated EUHT system can satisfythe requirements of CBTC system in high-speed scenariofirstly. As we know, the most important QoS parameter forsafety critical traffics is transmission delay which includecommunication delay without handoff and handoff latency.Figure 6 shows the communication delay without handoffand the handoff latency performance for these traffics. Aswe can see, when handoff does not happen, the averagecommunication delay without handoff is 5ms, 5ms, and6ms for CBTC, train state monitoring traffic, and emergencytext, respectively. The maximum values are 33ms, 55ms, and103ms for them, respectively. Therefore, the communicationdelay without handoff of safety-critical traffics can satisfythe requirements shown in Table 2 very well in high-speedscenario.

Moreover, as is shown in Figure 6, comparing with thecommunication delay without handoff, the maximum hand-off latency of CBTC, train state monitoring, and emergencytext can reach 146ms, 144ms, and 150ms. Even so, all thehandoff latency still satisfy the requirement of CBTC safetycritical traffics in high-speed scenario.

Figure 7 describes the throughput testing results ofthroughput critical traffics. In our test, 2-wayCCTVanaloguevideos, 2Mbps for each, and 1-way 8MbpsPIS analogue videoare transmitted in this integrated system.Thus, as is shown inFigure 7, because of the handoff, there are some fluctuationsin the throughput results of CCTV and PIS, but the averagevalue of CCTV and PIS all stabilize around the throughputrequirements presented in Table 2.

Something needs to be pointed out; when handoff hap-pens, the EAU will be disconnected with EBU. During thisperiod, the data cannot transmit between EAU and EBU. Asis shown in Figure 7, the throughput will be decreased during

8 Wireless Communications and Mobile Computing

Table 2: The requirement of different traffics in integrated urban rail transit.

Application Throughput uplink Throughput downlink Delay Handoff latency ReliabilityCBTC 100 kbps 100 kbps 150ms 150ms HighTrain state monitor 100 kbps None 150ms 150ms HighEmergency text None 100 kbps 150ms 150ms HighCCTV 4Mbps None 500ms 500ms MediumPIS None 8Mbps 500ms 500ms Low

0 5 10 15 20 25 30

Thro

ughp

ut (M

bps)

0

2

4

6

8

10

CCTV1CCTV2PIS

Elapsed time (minutes)

Figure 7: Test result of throughput of PIS and CCTV based onEUHT.

the handoff procedure. When the EAU completes the IFHOprocedure, it connects with new EBU. The untransmissiondatawill be transmitted in a short period.The throughputwillbe increased after the IFHO.

After the analysis of EUHT test results, we will show thecomparison with the TD-LTE test results which we get in theCircular Railway Experiment Station of China Academy ofRailway Sciences [16, 17]. Figures 8 and 9 are the test resultsof TD-LTE under the scenario with 200 km/h. As we can see,the maximum handoff latency of safety-critical traffics canreach 225ms which is much larger than the requirement inTable 2. Moreover, the throughput results can only satisfy therequirements with 1-way 1Mbps CCTV analogue videos and1-way 2Mbps PIS analogue video.

Therefore, compared with TD-LTE, the integrated EUHTwireless communication system can achieve better perfor-mance in safety-critical and throughput-critical traffics inhigh-speed scenario.

6. Conclusion

In this paper, we first detailed presented the structure ofintegrated EUHT wireless communication system based onurban rail transit and described all the traffics of tradi-tional CBTC system. Then, the theoretical values includingcommunication delay without handoff, handoff latency, andthroughput were calculated in ideal condition. In order to getthe real EUHTwireless communication system performance,an outdoor test scenario in Beijing-Tianjin High-Speed Rail-way was set up. Plenty of test results and the comparison

0 1 2 3 4 5 6 7 8 9 10

Resp

onse

tim

e (s)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

CBTCEmergency textTrain state monitor

Elapsed time (minutes)

Figure 8: Test result of communication delay and handoff latencybased on TD-LTE.

0

0.5

1

1.5

2

2.5

3

Thro

ughp

ut (M

bps)

PISCCTV

0 1 2 3 4 5 6 7 8 9 10Elapsed time (minutes)

Figure 9: Test result of throughput of PIS and CCTV based on TD-LTE.

with TD-LTE were shown in this paper. As we can see, notonly can the QoS performance of integrated EUHT wirelesscommunication system based on urban rail transit satisfy therequirement of safety critical traffics and throughput critical

Wireless Communications and Mobile Computing 9

traffics in high-speed scenario well, but also it has betterperformance than TD-LTE in the high-speed scenario.

Conflicts of Interest

The authors declare that there are no conflicts of interestregarding the publication of this paper.

Acknowledgments

This paper was supported by grants from BeijingNatural Science Foundation (L171004, I18E300010, andKIE017001531), the National Natural Science Foundation ofChina (no. 61603026), ChinaRailway (KID16001531), projects(RCS2016ZT011, RCS2017ZT006, and KIK17006531), and theBeijing Key Laboratory of Urban Rail Transit Automationand Control.

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